under the banner of something

Từ điển Oxford Advanced Learner 8th

under the banner (of sth) idiom
(formal)
1. claiming to support a particular set of ideas
•They fought the election under the banner of ‘No new taxes’.
2. as part of a particular group or organization
•Troops are in the country under the banner of the United Nations.
Main entry: ↑banner_idiom